PROJECT SUMMARY Aspergillus fumigatus is the most common etiologic agent of invasive aspergillosis, a devastating infection in patients with hematologic malignancies and in hematopoietic cell transplant recipients. Classically, patients with prolonged quantitative or qualitative defects in neutrophil and/or monocyte function are at risk for invasive aspergillosis. However, recent advances in targeted lymphoma therapies have resulted in new at-risk patient groups, particularly if a novel agent targets a signal transduction pathway implicated in cancer cell proliferation as well as immune surveillance. In a clinical study, we identified ibrutinib (IBT) as a risk factor for invasive fungal infections, and invasive aspergillosis in particular. Although IBT targets Bruton?s tyrosine kinase (BTK) to interrupt tonic B cell receptor signaling in malignant cells, IBT therapy leads to a defect in fungal immune surveillance, though the underlying molecular and cellular mechanisms remain undefined. BTK knockout mice are susceptible to A. fumigatus challenge and preliminary studies in mice and in human neutrophils suggest that IBT disrupts myeloid cell antifungal activity, in part by interfering with fungal cell phagocytosis. A critical knowledge gap relates to BTK function in myeloid cells and its role in mediating sterilizing antifungal immunity. The central hypothesis that underlies this proposal is that BTK signaling in myeloid cells is essential to couple A. fumigatus recognition to innate immune activation and fungal eradication. This model predicts that IBT impairs this immune surveillance function independent of its effects on B cells. To examine this model, we harness a unique fluorescent fungal reporter strain that monitors fungal uptake and viability during cellular encounters with murine leukocytes in the lung and human leukocytes in vitro and utilize gene knockout mice and a conditional gene targeting strategy to target myeloid cells that respond to, engulf, and kill conidia. We explore this model in the following aims: (1) define the fungal immune surveillance function and essential cellular source of BTK signaling in the lung, and (2) define the fungal immune surveillance defect caused by IBT administration in the lung and in human leukocytes. The proposed studies are significant and innovative because they translate an unexpected clinical observation into a systematic approach to decipher how a novel precision pharmaceutical compound interferes with a central pathway of fungal immune surveillance. Insight into the IBT-dependent fungal immune surveillance defect is likely to inform screening and prophylactic strategies for at-risk patients.